The present invention discloses a temperature-operated shape memory alloy actuator which can be used for appliance control. The actuator takes advantage of the R-phase of the alloy to confine the actuating temperature to a narrow operating range.
Temperature controlled actuators are known, and are usually based on the thermal expansion characteristics of various materials, especially metals. Bi-metal strips, in which the different expansion characteristics of different metals causes the strip to curl, are especially popular. Shape memory alloys (SMA) can also be used in various control applications due to their unusual thermal characteristics.
SMA elements are fabricated with a high temperature xe2x80x9cmemoryxe2x80x9d shape. When cooled below the defined transition temperature, the SMA element will retain its high temperature shape under no-load conditions. However, when a load is applied to the SMA element, the SMA element will be deformed. If the load is subsequently removed, the deformation will remain. When the SMA element is then heated above its defined transition temperature, it will return to its original xe2x80x98memorizedxe2x80x99 shape. SMA elements are worked to the desired size and then a heat treatment process is necessary to set the high temperature shape of the material. Transformation temperatures increase with increasing heat treatment temperatures. When the alloy is in the xe2x80x9cmartensiticxe2x80x9d phase, below its transition temperature, the material can be easily deformed when a load is applied to it. The alloy will remain in the deformed shape until the alloy is heated above its transformation temperature and changes to its xe2x80x9causteniticxe2x80x9d state, whereupon it xe2x80x9cremembersxe2x80x9d its set shape. Upon cooling below the transformation temperature, the alloy returns to its deformed shape. This is a different phenomenon than the normal thermal expansion and contraction of metal, and occurs because SMA materials have two different phases, with a different crystal lattice structure in each. However, SMA materials exhibit a hysteresis effect, and the xe2x80x9cwarmingxe2x80x9d transition temperature is generally higher than the xe2x80x9ccoolingxe2x80x9d transition temperature. The difference in these two transition temperatures is typically between 10-50 deg C, resulting in a wide temperature band in which the material will remain in its current phase, regardless of which phase that might be.
One application of SMA materials involves their use in springs whose spring rate (the force resisting compression or tension) is different in the austenitic phase than in the martensitic phase. Such SMA springs can be used in temperature-sensing mechanical actuators, with actuation being caused when the spring transitions between its two phases because the ambient temperature (and therefore the temperature of the SMA material) passes through the transition temperature. Examples of this application can be found in U.S. Pat. Nos. 4,522,219 and 4,523,605. Unfortunately, accurate bi-directional control with such devices is difficult because the aforementioned hysteresis effect tends to make the device unresponsive in the relatively wide range between the warming and cooling transition temperatures.
A typical use of an SMA spring actuator is to open a drain plug in a cooling system when ambient temperatures approach freezing. Cooling systems such as air conditioners typically condense water vapor out of the air, and collect it in a drain pan. During normal operation, the water in the pan may be picked up by a xe2x80x98slingerxe2x80x99 attached to the condenser fan and propelled against the condenser coils to increase heat transfer efficiency. But if ambient temperatures drop below freezing, the water can freeze, locking up the condenser fan and causing various other problems. An anti-freeze drain hole can be opened when temperatures approach freezing, allowing the water to drain away before it can freeze and cause any problems. The non-electrical nature of an SMA actuator makes it ideal for this application because it does not create a shock hazard. Unfortunately, the wide hysteresis effect of SMA actuators can cause the drain plug to be in an unpredictable state over a wide temperature range, leading to inefficiencies in the cooling system.
A temperature-sensitive SMA actuator is needed that operates bi-directionally over a much smaller temperature range than conventional SMA actuators, so that more precise temperature-related control can be achieved.
The actuator of the present invention can overcome the aforementioned problems by taking advantage of a characteristic of SMA materials called the xe2x80x9cR-phasexe2x80x9d. The R-phase is a martensitic-like phase which, under certain conditions, is present on cooling before the alloy enters the martensitic phase. Under these conditions, the alloy""s structure begins to change at a temperature (the xe2x80x9cR-thresholdxe2x80x9d) which can be several degrees higher than the normal cooling threshold, and may be within 1.5 deg C of the warming threshold temperature. Upon entering this phase, the crystal structure begins to weaken and change, but does not fully change into the martensitic phase until the temperature drops below the normal cooling threshold temperature. As the alloy enters the R-phase from the austenitic phase, and provided the material does not enter the martensitic phase, very tight temperature hysteresis can be achieved. The R-phase only occurs upon cooling, and only occurs when the recoverable strain in the SMA material is less than a predetermined amount, typically less than about 1%. Recoverable strain is a measure of the difference between the strain in the SMA spring in the austenitic phase in one position and a non-austenitic phase in the other position. When the actuator is thus configured so that the recoverable strain in the SMA spring is within the proper limits to permit creating an R-phase, the R-phase of the SMA material is said to be enabled. Actually entering the R-phase then depends on cooling the SMA spring below the R-threshold.
The actuator of the invention can include opposing springs: an actuating spring of SMA material and a biasing spring of standard spring material such as spring steel. A preferred embodiment places both springs in compression, but the actuator can also be configured with both springs under tension. The forces of the opposing springs can be balanced so that the force of the SMA spring in its strong state is greater than that of the biasing spring, but in its weak state is less than that of the biasing spring. Thus the position of an actuating element can be controlled by the stronger spring, with the stronger spring being determined by the phase of the material in the SMA spring, which is in turn controlled by ambient temperature. By designing the actuator so that the SMA spring material will enter the R-phase during cooling, the hysteresis effect of the SMA material can be reduced to a small temperature range, such as less than 2 deg C.
In a preferred embodiment, the actuator may be part of a cooling system in which the actuator seals a drain hole in a condensate pan with a plug when the temperature is above a predefined threshold temperature, such as 5 deg C, and removes the plug from the drain hole to drain the condensate when the temperature is below that threshold.